He worries that Banquo will be a threat because he suspects something (as he does) or because he will have sons who will be kings. So Macbeth arranges murderers to kill Banquo and his son Fleance. Note that Macbeth also fears the murderers will betray him, and even hires a third murderer to keep the other two in check.

In Act 3, Scene 4 Macbeth loses it. He thinks he sees Banquo’s ghost, and he cannot control himself even to act like things are normal in front of his guests. They try to laugh it off, but Lady Macbeth knows it’s not good for Macbeth’s image.

So while Macbeth is worried, he is still acting to control his fate. He wants to keep his throne at all costs, including murder. He does not seem to care how many murders he commits. His fear is the reason he decides to seek the Weird Sisters out again, and see what else the witches can tell him. Big mistake!